1. Field of the Invention
The present invention relates to sailboard fins, and more particularly to an improved fin having a pair of foils in tandem.
2. Description of the Prior Art
Within the past few years, the sport of windsurfing has become very popular. A board, similar to a surfboard, is utilized with a sail structure attached to the upper deck of the board. The board sailor stands on the board and grasps a frame attached to the sail support. The sailor thereby controls the position of the sail with respect to the wind direction and the board to sail in a desired path. Speeds on the order of 35 knots are achieved with current equipment and speed sailing competition is popular. Another aspect of windsurfing involves a variety of sharp turning maneuvers such as performed in slalom racing and waveriding (surfing), and jumping in which the entire board leaves the water.
A key element of the sailboard is a fin which projects downward from the underside and at the rear of the board. The fin is a small foil having cambered surfaces. The sail produces a tendency to move the board sideways through the water. The purpose of the fin is to counteract the sail's force to produce a forward motion of the board. The force produced by a foil moving through a fluid can be expressed mathematically as ##EQU1## where F.sub.1 is the lifting force of the foil,
C.sub.L is the coefficient of lift of the foil, PA1 d is the density of the fluid. PA1 A is the area of the foil, and PA1 v is the velocity of the foil relative to the fluid.
C.sub.L is a function of the angle of attacke of the foil, and its configuration. For a fixed shape foil and angle of attack, the force is seen to be proportional to the area, to the fluid density, and to the velocity squared. As will be recognized, a small fin can counteract the force of a large sail due to the large difference in densities between air and water. Another consideration is the velocity since the force increases by the square of the velocity.
Prior art fins have been constructed having a wide variety of shapes. The span is the length of a fin from the tip to the root or base and the chord length is the width at the root. Generally, the span is long relative to the chord length. The span length to chord length is termed the aspect ratio. A low aspect ratio will permit a larger area for a given span. A high aspect ratio, of course, requires a greater span to obtain the same area. Fins generally are swept back, having the tip behind the base.
A phenomenum known as "spinout" is common with conventional high aspect ratio fins commonly used for high speeds and sharp maneuvering. There appear to be two actions responsible for spinout: stall and ventilation. Stall occurs when the angle of attack is excessive, producing turbulence in the flow of water across the fin and loss of lift. Ventilation occurs when air is drawn from the surface of the water into the low pressure or lifting region of the fin and may be due to spanwise flow of water. That is, water tending to flow from root to tip will tend to draw the air spanwise under certain conditions.
Spanwise flow is highly dependent upon the design parameters of a fin such as the amount of taper of the foil, the sweep, the aspect ratio, and the thickness. Spanwise flow has been reduced in some fins by using a cutout at the root which produces a reverse taper of the fin. However, there is a loss of efficiency with such planforms.
There is a need for a fin design which will avoid stall at large angles of attack, will minimize ventilation, and will have a large area while maintaining a high aspect ratio to reduce spanwise flow, yet which will maintain high efficiency.